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Amide bond formation: beyond the myth of coupling reagents
Eric Valeur*w and Mark Bradley*
Received 23rd June 2008
First published as an Advance Article on the web 4th December 2008
DOI: 10.1039/b701677h
Amide bond formation is a fundamentally important reaction in organic synthesis, and is
typically mediated by one of a myriad of so-called coupling reagents. This critical review is
focussed on the most recently developed coupling reagents with particular attention paid to the
pros and cons of the plethora of ‘‘acronym’’ based reagents. It aims to demystify the process
allowing the chemist to make a sensible and educated choice when carrying out an amide
coupling reaction (179 references).
Introduction
Amide bonds play a major role in the elaboration and
composition of biological systems, representing for example
the main chemical bonds that link amino acid building blocks
together to give proteins. Amide bonds are not limited to
biological systems and are indeed present in a huge array of
molecules, including major marketed drugs. For example,
Atorvastatin 1, the top selling drug worldwide since 2003,
blocks the production of cholesterol and contains an amide
bond (Fig. 1),1 as do Lisinopril 2 (inhibitor of angiotensin
converting enzyme),2 Valsartan 3 (blockade of angiotensin-II
receptors),3 and Diltiazem 4 (calcium channel blocker used in
the treatment of angina and hypertension).4
Amide bonds are typically synthesised from the union of
carboxylic acids and amines; however, the unification of these
two functional groups does not occur spontaneously at
ambient temperature, with the necessary elimination of water
only taking place at high temperatures (e.g. 4200 1C),5
conditions typically detrimental to the integrity of the
substrates. For this reason, it is usually necessary to first
activate the carboxylic acid, a process that usually takes place
by converting the –OH of the acid into a good leaving group
prior to treatment with the amine (Scheme 1). Enzymatic
catalysis has also been investigated for the mild synthesis of
amides and the organic chemist may find some of these
methods useful as an alternative to traditional methods.6,7
In order to activate carboxylic acids, one can use so-called
coupling reagents, which act as stand-alone reagents to
generate compounds such as acid chlorides, (mixed) anhydrides,
carbonic anhydrides or active esters. The choice of coupling
reagent is however critical. For example, in medicinal
chemistry library-based synthesis, amides are often generated
using broad ranges of substrates with varying reactivities
(e.g. anilines, secondary amines, bulky substrates). A coupling
reagent needs to be able to cope with this whole portfolio of
reactivity. Many reviews on coupling reagents have been
published,8–14 illustrating their importance in the synthetic
armoury of the synthetic chemist, but these reviews have often
failed to offer a critical view on the subject making the choice
of reagent difficult. An important issue is that many of the
coupling reagents reported have not been compared to others,
making any real evaluation impossible. As many groups have
reported ‘‘new’’ reagents as being wonderful and better than
others, the chemist looking at the field of coupling reagent for
University of Edinburgh, School of Chemistry, West Mains Road,Edinburgh, UK EH9 3JJ. E-mail: mark.bradley@ed.ac.uk;Fax: (+44) 131 650 6453; Tel: (+44) 131 650 4820
Eric Valeur
Eric Valeur obtained hisPh.D. under the guidance ofProf. Bradley at the Univer-sity of Edinburgh in 2005, andworked as a Postdoctoralfellow at the Northern Insti-tute for Cancer Research,Newcastle, in Prof. Griffin’sgroup. He then joined Merck-Serono in France, beforemoving recently to Novartis,within the medicinal chemistrygroup of the ExpertiseProtease Platform.
Mark Bradley
Professor Bradley’s researchinterests are focused on theapplication of the tools andtechniques of chemistry toaddress biological problemsand needs, typically with ahigh-throughput twist. Twothemes dominate at this time:the development of non-DNAbased microarray platformsfor cell and enzymatic basedassays and the development ofchemistries that enable effi-cient cellular delivery of pro-teins, nucleic acids, sensorsand small molecules.
w Present address: Novartis Pharma AG, FAB-16.4.06.6, CH-4002Basel, Switzerland. evaleur@yahoo.fr
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CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews
the first time can be completely lost. The process can be made
even more complicated as epimerisation, usually through an
oxazoline intermediate, may take place during amide bond
formation. Thus, when coupling reagents are evaluated,
several tests that have been developed to assess the extent of
epimerisation (see Table 1) should be carried out.
1. Coupling using carbodiimides
1.1 Dicyclohexylcarbodiimide
Carbodiimides were the first coupling reagents to be synthe-
sised. Dicyclohexylcarbodiimide (DCC, 5) has been used for
coupling since 1955,21 and the mechanism for coupling
carboxylic acids to amines is shown in Scheme 2.
The first step involves the reaction of the carboxylic acid
with DCC to form the O-acylurea 6. This intermediate can
then yield a number of different products:
� The amide via direct coupling with the amine (the
by-product formed, dicyclohexylurea (DCU 7), is usually
insoluble in the reaction solvent and can be removed via
filtration).
� Formation of an N-acylurea 8 by-product
� Formation of the carboxylic acid anhydride which
subsequently yields the amide by reaction with the amine
(needs 2 equiv. of acid).
When using DCC, oxazolone formation can take place after
generation of the O-acylurea leading to epimerisation,19
especially important when activating acid groups in the aposition of an amide bond.
In addition to peptide synthesis, carbodiimides (often
with N-hydroxysuccinimide as an additive) have been used
extensively in nanotechnology for the functionalisation of
monolayers on surfaces and nanoparticles.22,23
1.2 Use of additives
In order to reduce the epimerisation level when using carbo-
diimides as coupling reagents, Koenig and Geiger introduced
1-hydroxy-1H-benzotriazole (HOBt) 9 as an additive,24,25
showing that, when using this additive, yields were higher
and epimerisation levels lower. For example, when coupling
Z-Gly-Phe-OH to H-Val-OMe, the epimerisation levels
dropped from 35% to 1.5%.
HOBt 9 is believed to work by initially reacting with the
O-acylurea 6 to give the OBt active ester 10, which enhances the
reactivity of the ‘‘activated ester’’ by encouraging/stabilising
the approach of the amine via hydrogen bonding (Scheme 3).
However, HOBt can yield by-products, thus it catalyses the
formation of diazetidine 11 (Scheme 4).26
In 1994, Carpino reported a related additive, 1-hydroxy-
7-azabenzotriazole (HOAt) 12 (Fig. 2), which was even more
efficient than HOBt 9 in terms of yield, kinetics and reduced
epimerisation levels.27 For example epimerisation during
coupling of Z-Val-OH and H-Val-OMe using DCC 5 dropped
from 41.9% with HOBt 9 to 14.9% with HOAt 12, while
during the coupling of Z-PheVal-OH to H-Ala-OMe using
Fig. 1 Examples of top drugs containing an amide bond. These
examples are just a small selection of drugs containing amide bonds
illustrating the importance of this functional group.
Scheme 1 Principle of the activation process for amide-bond
formation.
Table 1 Common epimerisation tests used for coupling reagent evaluation involving amino acids
Entry Author Acid Amine Analysis method
1 Young15 Z-Leu-OH H-Gly-OEt Optical rotation2 Weinstein16 Ac-Phe-OH H-Ala-OMe NMR3 Bodansky17 Ac-isoLeu-OH H-Gly-OMe Chiral HPLC4 Anteunis18 Z-Gly-Phe-OH H-Val-OMe HPLC or NMR5 Anderson19 Z-Gly-Phe-OH H-Gly-OEt Fractional crystallisation6 Izumiya20 Z-Gly-Ala-OH H-Leu-OBz Hydrogenation followed by HPLC
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EDC, it dropped from 4.1% with HOBt 9 to under 2% with
HOAt 12.27
Much work has been carried out on the benefit of using
additives. In particular, Carpino studied various isomers of
HOAt concluding that the 7-isomer was the most efficient.28
Albericio also showed that copper(II) complexes with
HOAt 11 or HOBt 9 were efficient additives in lowering the
epimerisation level.29
However, safety considerations when using benzotriazoles
(and variants) need to be carefully considered as these
compounds display explosive properties.30,31
1.3. Other carbodiimides
Since the application of DCC to amide bond formation, many
carbodiimides, including DIC 13 (diisopropylcarbodiimide),
have been reported and this field has been reviewed.26 In
particular, attention has focused on so-called water-soluble
carbodiimides, as the ureas formed when using DCC 5 or the
popular diisopropylcarbodiimide DIC 13 can sometimes be
difficult to remove. Sheehan investigated several derivatives
14–17, and concluded that coupling was more efficient when
using tertiary amine carbodiimides rather than quaternary
derivatives (e.g. 14 4 16).32,33
Carpino compared DIC 13 to EDC 20 and analogues
18–19,34 and also compared DIC 13 to some unsymmetrical
alkyl/aryl carbodiimides such as phenyl ethyl carbodiimide
(PEC 21) and phenyl isopropyl carbodiimide (PIC 22) (Fig. 3,
Table 2). Overall, when using HOAt as an additive, DIC gave
the best results for peptide segment coupling.
Other carbodiimides, BMC 23 and BEC 24 have been proposed
by Izdebski, but these reagents showed no benefit over DIC 13.35
Another so-called ‘‘water extractable’’ carbodiimide, BDDC
25 was synthesised and its efficiency was comparable to DCC 5
and EDC 20.36
2. Coupling reagents based on 1H-benzotriazole
Several ‘‘salts’’ are often associated with reagents based on
1H-benzotriazoles, including uronium/aminium, phosphonium
and immonium salts (Fig. 4).
Scheme 2 Coupling using DCC.
Scheme 3 Mechanism of activation by 1-hydroxy-1H-benzotriazole
when used as an additive with DCC.
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2.1 Uronium/aminium salts
Many coupling reagents are based on the HOBt/HOAt system
and uronium/aminium salts.37 Uronium 26 and aminium
27 isomers of these reagents have been structurally identified
and the true forms is probably a mater of debate depending on
solvent, isolation method and counter anion etc. (Fig. 5).38
Coupling reagents based on uronium salts were first reported
as the O-isomer (26). However, Carpino showed by X-ray
crystallography that HATU 28a and HBTU 28b were in fact
the N-isomer (27).38
These reagents react with carboxylic acids to form OAt/OBt
active esters, which then react with amines (Scheme 5).
A side-reaction can often take place with the amine reacting
with the coupling reagent to form a guanidinium by-product
29 (Scheme 6),14 thus order of addition and timing are crucial.
Comparative studies using HBTU39 28b and TBTU40 30b
showed that the counter-anion had no practical influence on
the outcome of coupling reactions using these reagents
(Fig. 6). Carpino showed that the best results were obtained
with HOAt, and many coupling reagents started to be based
on this additive such as HATU 28a and TATU 30a.27 It
has been proven that coupling reagents based on HOAt
(compared to HOBt) give faster, more efficient couplings with
less epimerisation.41 Much work has been carried out with
variation of the substituents, yielding HAPyU 31 (also named
BBC by Chen42) and TAPipU 32 with relatively little impact
on the outcome of couplings.43 Other modifications include
HAPipU3733a, HBPipU44
33b, HAMDU3734a, HBMDU37
34b (also named BOI), and HAMTU3735. Overall the
structural differences between these reagents did not appear
to be based on rational considerations and were merely a
screening of different substituents. Reagents 33–35 gave poor
coupling results because the reagents were too reactive and
degraded before coupling could take place.
Carpino modified the HOAt ring to form 5,6-benzo (36) and
4,5-benzo (37) derivatives,45 which showed no real benefit over
classical methods. In fact when used as additives with DIC, the
epimerisation was higher than when using HOAt as additive.
More recently, derivatives HCTU 40a and TCTU 40b based
on 6-chloro-HOBt were developed by Albericio,46 but these
reagents have not been directly compared to other coupling
reagents.
Scientists at Argonaut also reported a 6-chloro-HOBt-based
reagent, ACTU 40c,47 which was compared to DIC 13. Some
results were very disappointing as a simple, unhindered acid
(phenylacetic acid) was only activated to 36%. This result was
only improved to 70% when using an excess of acid, demon-
strating that ACTU is a fairly poor coupling reagent.
Recently El-Faham developed some new reagents based on
‘‘immonium salts’’.48 However, according to the terminology
used in coupling reagents, these belong to the aminium/
uronium salt-based class. Based on HOAt-/HOBt-rings,
HAM2PyU 41a, HBM2PyU 41b, HAM2PipU 42a, HBM2PipU
42b, HAE2PyU 43a, HBE2PyU 43b, HAE2PipU 44a,
HBE2PipU 44b, HATeU 45a and HBTeU 45b were synthe-
sised. El-Faham firstly investigated the stability of these new
reagents both in solution and in the solid state. Solids and
solutions (in DMF) were stable for 3–4 weeks when kept under
an inert atmosphere. However, like most coupling reagents,
the reagents degraded rapidly when left in solution in the
presence of a base. Thus, coupling involving hindered or
poorly reactive substrates can be expected to be poor as longer
reaction time are typically required for these substrates.
Efficiency of the reagents was tested by measuring the
Scheme 4 Formation of the diazetidine by-product when using DCC/HOBt.
Fig. 2 Structure of 1-hydroxy-7-azabenzotriazole.
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half-life of the activated esters of Z-Aib-OH in the presence of
4-chloroaniline. HOAt-based reagents HAM2PyU 41a,
HAM2PipU 42a, HAE2PyU 43a, HAE2PipU 44a, HATeU
45a reacted more quickly than the HOBt-based reagents
HBM2PyU 41b, HBM2PipU 42b, HBE2PyU 43b, HBE2PipU
44b, HBTeU 45b. However no yields were given, which makes
the direct comparison of the reagents impossible. Indeed, the
activated esters might be hydrolysed rather than coupled to
the poorly nucleophilic 4-chloroaniline. Epimerisation was
low (Anteunis test) when the reagents were used in the
presence of collidine but was as high as 11.8% in the presence
of DIPEA when using HBTeU 45b. Overall it was not evident
Fig. 3 Structure of some common carbodiimides.
Table 2 Results obtained when coupling Z-Phe-Val-OH toH-Pro-NH2 with various carbodiimides and HOAt as an additive34
Entry Coupling reagent Yield (%) LDL (%)
1 DIC 86 2.12 PEC 91 5.63 PIC 89 9.64 EDC 85 4.75 EDC�HCl 81 4.1
Fig. 4 Salts associated with reagents based on 1H-benzotriazole.
Fig. 5 Aminium and uronium isomers.
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that any of the new reagents reported were beneficial over a
reagent like HATU 28a.
Recently, El-Faham reported further development of such
coupling reagents.49 HDMA 46a, HDMB 46b, and 6-HDMCB
47 were evaluated and little variation on epimerisation levels was
noticed, but HDMA 46a proved to give higher yields for the
synthesis of Fmoc-Val-Val-NH2 compared to HATU 28a. Other
reagents such as 6-HDMFB 48, 4-HDMA 49, HDMTA 50a and
HDMTB 50b were also synthesised.50 Overall there was hardly
any difference between the different reagents. HDMB 46b
displayed the best hydrolytic stability while having better solubility
than HATU 28a. Morpholino derivatives HDMA 46a and
HDMB 46b showed better efficiency than their thio analogues
HDMTA 50a and HDMTB 50b.
2.2 Phosphonium salts
Another family of coupling reagents based on HOBt/HOAt
uses a phosphonium group. Phosphonium salts have the
advantage of not yielding guanidinium by-products via reac-
tion of the coupling reagent with amines. The first HOBt/
HOAt-phosphonium salt introduced was BOP 51b,51 but its
use has been limited due to the carcinogenicity and respiratory
toxicity associated with HMPA generated when BOP 51b is
used in coupling reactions, leading to the development of the
pyrrolidino derivative PyBOP 52b.52 Carpino prepared AOP37
51a and PyAOP37,53 52a and compared them to BOP 51b and
PyBOP 52b, and showed that the aza-derivatives were more
reactive.
For the synthesis of thioamides, Hoeg-Jensen developed
phosphonium coupling reagents based on 6-nitro HOBt
(Fig. 7).54 PyNOP 53, PyFOP 54 and NOP 55 were used
successfully for the formation of thioamides, with good thioamide/
amide selectivity but their solubility in organic solvents was
poor. Moreover, the results obtained with PyBOP were very
similar to PyNOP 53, PyFOP 54 and NOP 55.
In a recent patent, PyClock 56 was disclosed as a new
coupling reagent.55 However hydrolysis was shown to be
worse than PyBOP 52b in the absence of base after 6 h and
this was also worse in the presence of a tertiary base as around
88% had been hydrolysed after 1 h compared to 81% for
PyBOP 52b under these conditions. The efficiency of PyClock
56 was evaluated via the solid-phase synthesis of three
pentapeptides which incorporated hindered/N-methylated
aminoacids (Table 3).
2.3 Immonium salts
Li designed and synthesised immonium/carbonium type cou-
pling reagents,56,57 such as BOMI 57,56,58–61 BDMP 58,56,60,61
BPMP 59, BMMP 60, and AOMP56,5961 (Fig. 8). BOMI 57
and BDMP 58 showed the best results, achieving 490%
conversion within 10 min during the coupling of Z-Gly-Phe-OH
with H-Val-OMe (Anteunis test). In addition, epimerisation
was low, BOMI 57 displaying 3.1% and BDMP 58 2.3% of
the DL-isomer. However, these reagents were not compared to
classic reagents such as HATU 28a or PyBOP 52b. As an
application, these reagents were used to carry out the total
synthesis of Cyclosporine O, an immunosuppressive agent.62
2.4 Other reagents
DepOBt (originally called BDP) 62b was reported by Kim
(Fig. 9).63 The reagent appeared to couple aniline to benzoic
acid or phenylacetic acid in high yield, and also aminoacids
(Phe, Val, Met, Ile) to other amino acids (Gly, Ser, Val) in high
yield although N-Methylated substrates were not tested.
Epimerisation was evaluated via Young’s test and found to
be low. The same group reported DpopOBt 63b but epimeri-
sation was high.64
Carpino reported DepOAt 62a, DpopOAt 53a, DmppOAt
64, DtpOAt 65a and DtpOBt 65b.65 Again, no real improve-
ment was gained compared to HATU 33a. For the synthesis of
ACP(65-74), HATU 33a outperformed any of these reagents.
An epimerisation study for the coupling of Z-Phe-Val-OH and
H-Pro-NH2 showed that DmppOAt 64 (3.6% of LDL isomer)
and DtpOAt 65a (2.9%) gave less epimerisation than HATU
28a (5.0%), while DtpOBt 65b was worse (11.4%), but no
explanation was given.
HAPyTU 66, a thio-analogue of HAPyU 31, was tested by
Klose but proved to be unsuccessful as yields were lower and
epimerisation higher than HAPyU 31.66
Another type of reagent based on sulfonates was developed
by Itoh.67 These reagents 67–70 incorporated HOBt or HOCt
(6-chloro-HOBt) with different substituents on the sulfonate.
The best results were obtained with HCSCP 70, the chlorine
group enhancing the reactivity of the reagent. However, the
reagents were not compared directly to each other. Compared
to DCC 5 (without using HOBt), these reagents gave less
side-reactions and the by-products were easily removed during
aqueous workup. According to the authors, epimerisation was
Scheme 5 Activation process using uronium/aminium type reagents.
Scheme 6 Guanidinium formation with aminium/uronium type
coupling reagents.
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lower than with DCC 5, but this was no surprise as DCC alone
give very high levels of epimerisation.
2.5 Conclusion on 1H-benzotriazole-based reagents
1-H-benzotriazole-based reagents probably represent the
widest class of coupling reagents. Although differences in
reactivities have been reported by their authors, there is
practically very little difference, as exemplified by Hachman,68
and HBTU 28b or TBTU 30b are reagents which usually
perform very well. Surprisingly, the potential explosive prop-
erties of these reagents is almost always disregarded.30,31
3. Reagents generating acid halides
3.1 General reagents used in organic chemistry and triazine-type
reagents
Fischer reported the first synthesis of a dipeptide (Gly-Gly) in
1901 using acid chlorides for coupling.69 The general approach
consisted of using reagents such as thionyl chloride or phos-
phorus pentachloride to generate the acid chloride which
reacted quickly with amines to form amides. This original
method was quite harsh and not compatible with many
protecting groups. It has however been adapted by Carpino
to synthesise peptides via a Fmoc strategy.70 Triphosgene has
also been reported to generate amino-acid acid chlorides,71
especially useful for hindered substrates.72 Similarly, acid
cyanides and azides have been used to synthesise amides.73
Cyanuric fluoride 71 can be used to synthesise acid
fluorides,74 which couple N-methylated amino-acids very
efficiently. A variety of other reagents have been reported
for the formation of acid fluorides, and include Deoxo-Fluor
72 and DAST 73 (Fig. 10).75 However a side-reaction is
observed when using Deoxo-Fluor 72 especially with hindered
amines (Scheme 7), which limits the applicability of this
reagent. In addition, Deoxo-Fluor 72 and DAST 73 are
expensive and hazardous reagents, and purification by
chromatography is required after reaction.
Part of this category of reagents is based on triazines
(cyanuric fluoride, chloride and derivatives) and has been
reviewed in details by Kaminski.76 The mechanism of activa-
tion involves the generation of an acid halide moiety
Fig. 6 Uronium/Aminium-based coupling reagents.
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(Scheme 8). Thus CDMT 74 and DCMT 75 (2,4-dichloro-6-
methoxy-1,3,5-triazine) have been successfully applied in the
synthesis of acid anhydrides (Fig. 11).77
3.2 Halo-uronium and halo-phosphonium type reagents
(Fig. 12)
TFFH 76a,78 BTFFH 77,78,79 and DFIH7878a have been used
to generate acid fluorides with amino acids such as histidine
and arginine since the activated form of Fmoc-Arg-OH under-
went deactivation via lactam formation when using cyanuric
fluoride.78 PyFloP 79a did not yield any acid fluoride.78
Interestingly, TFFH 76a (100% coupling after 10 min) gave
better results than the analogues TCFH 76b (86%) and TBFH
76c (79%), for the coupling of Fmoc-Val-OH to H-Ile-PEG-PS,78
but overall, BTFFH 77 gave the best conversions.79
El-Faham synthesised three acid fluoride generating
reagents: DMFFH 80, DEFFH 81 and TEFFH 82,48 but
these were poorly stable to hydrolysis in the presence of a
base (most of the reagent hydrolysed within 1 h). The reactivity
of these reagents was studied by monitoring acid fluoride
formation for various hindered and unhindered amino acids,
and all three reagents were shown to be less reactive than
TFFH 76a or BTFFH 77.
Reagents aimed at generating acid chlorides or bromides
under milder conditions than thionyl chloride have been
targeted. BroP 83a was first synthesised by Coste,80 followed
by PyBroP 79b and PyCloP 79c.81 These reagents were shown
Fig. 10 Structure of Deoxo-Fluor 72 and DAST 73.
Fig. 7 Phosphonium type coupling reagents.
Table 3 Comparison of pentapeptides yield when using PyClock 56
and PyBOP 52b
Yield (%)
Entry Amine PyClock PyBOP
1 H-Tyr-NMeVal-Phe-Leu-NH2 11 02 H-Tyr-Aib-Aib-Phe-Leu-NH2 97 833 H-Tyr-Arg-Arg-Phe-Leu-NH2 85 75
Fig. 8 Immonium type coupling reagents.
Fig. 9 Other coupling reagents based on 1-hydroxybenzotriazole and
1-hydroxy-7-azabenzotriazole.
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to be more efficient that PyBOP 52b in couplingN-methylamino
acids. PyClU 84, also synthesised by Coste, gave high yields
when coupling hindered amino acids,81 while DCIH 78b
(named CIP originally) gave comparable results to PyBroP
79b and PyCloP 79c.82 One of the drawbacks of PyBroP 79b,
PyCloP 79c and DCIH 78b is the established formation
oxazolones. CloP 83b was reported by Castro and shown to
give low levels of epimerisation via Young’s test.83
PyClopP 85, an analogue of PyCloP 79c, was reported by Li
in an attempt to increase reactivity by replacing a pyrrolidine
ring with a phenyl group. The reagent was reported as being
efficient for hindered peptide synthesis, but no results were
given to illustrate this fact.57
BOP-Cl 86 is a reagent that has been widely used in peptide
synthesis,84 and was in particular reported as being suitable for
coupling hindered substrates,85 but it has the major drawback
of capping primary amines.86
Other reagents include CDTP87 87 and CMMM84 88, but
these reagents, like PyBroP 79b and PyCloP 79c, usually give
high epimerisation during coupling. CMMM 88 was also
compared to other reagents such as FEP 96b, and gave poor
results with coupling times of over 2 h and epimerisation of
over 30% (Anteunis test).57
DMC 89, has been investigated as a coupling reagent.88 It
proved to be successful in the generation of some amides but
questions of functional group compatibility are raised when
considering its high reactivity. Recently, El-Faham tested
DMFH 90a and DMCH 90b. DMFH 90a was really efficient
for coupling the hindered Aib amino acid to a tripeptide
Aib-Phe-Leu. The tetrapeptide was synthesised on solid phase
in 99% yield compared to 68% for HATU 28a,50 but complete
scope of this reagent was not investigated. DMCH 90b on the
other hand performed poorly.
3.3 Halo-sulfonium, halo-dioxolium and halo-dithiolium
coupling reagents
Li synthesised other types of coupling reagents, including
CDMS 91, CBDO 92 and CPDT 93 (Fig. 13).57 However
these reagents were far too reactive and decomposed in
solution before activation could take place.
3.4 Halo-thiaziolium and halo-pyridinium type reagents
Li designed reagents based on thiazolium and 2-halopyridinium
salts. Their design was based on the fact that, in halouronium
type coupling reagents, the carbocation is well stabilised via
the electron pairs on the amine groups. Therefore, the
carbocation shares a relatively high electron density and the
uronium salt demonstrates relatively low reactivity in
the addition of the carboxylic acid. For this reason Li
attempted to replace one nitrogen group with other groups
without lone pairs or more electronegative groups with lone
pairs to enhance the reactivity of the reaction-mediated
carbocations. The first attempt to replace nitrogen with sulfur
yielded thiazolium reagent, BEMT 94.89 The same type of
Scheme 7 Side-reaction observed during the activation process when using Deoxo-Fluor.
Scheme 8 Formation of acid halides when using triazines as coupling
reagents.
Fig. 11 Coupling reagents based on triazines.
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reagent, BMTB 95, was proposed by Wischnat (Scheme 9).90
BMTB 95 performed better than HATU 28a in coupling
Boc-N(Me)-Ile to N(Me)-Ile-OBn. However BMTB 95 was
not compared to BEMT 94.
Li reported 2-halopyridinium salts such as BEP 96a, FEP
96b, BEPH 97a and FEPH 97b (Fig. 14).91 Mukaiyama has
extensively used 2-chloro- and 2-bromo-pyridinium iodide 98
to synthesise esters, lactones and amides,92 but the conditions
used were not ideal for peptide synthesis, as reactions had to
be performed at reflux in DCM due to the poor solubility of
the reagents. For this reason Li used tetrafluoroborate and
hexachloroantimonate counter anions to improve solubility,
and chose the fluoro-analogues for higher reactivity. The
efficiency of these reagents proved to be higher than BTFFH
77, PyBrop 79b, PyClU 84 or BOP-Cl 86. However these
reagents might be a bit too reactive as the base used during the
coupling had to be added very slowly to avoid the coupling
reagents reacting too violently. Thus side-reactions may be
expected for some substrates.
4. Other coupling reagents
4.1 Reagents generating carbonic anhydrides (Fig. 15)
EEDQ 99, was originally developed in 1967.93 EEDQ 99 offers
several advantages over most coupling reagents, as the
reaction with an amine cannot yield a guanidinium salt, a
typical side reaction observed with uronium type coupling
Fig. 12 Halo-uronium and halo-phosphonium type reagents.
Fig. 13 Halo-sulfonium, halo-dioxolium and halo-dithiolium type
reagents.
Scheme 9 Synthesis of BEMT and BMTB.
Fig. 14 Halo-pyridinium type reagents.
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reagents. In addition, the carbonic anhydride is formed slowly
but consumed rapidly, which avoids its accumulation and
therefore minimises the possibility of side-reactions such as
epimerisation, and it can also be used with unprotected
hydroxy residues.93 EEDQ 99 has thus been used for the
synthesis of various amide derivatives.94,95 Analogues of
EEDQ 99 have also been successfully investigated such as
IIDQ 100, and a number of unsymmetrical reagents.96 Not
many comparison studies have been published, but IIDQ 100
proved, over a few examples, to perform slightly better than
EEDQ 99 (Table 4).97 Interestingly, when compared to other
coupling reagents without activation, IIDQ 100 outperformed
HATU 28a, PyAOP 52a and BOP-Cl 86.97
4.2 Triazine-based reagents (not generating acid halides)
DMTMM 101 is a triazine derivative, which has the particular
advantage of promoting amide synthesis in alcohols or aqu-
eous media, without ester formation and with selectivity
comparable to DCC 5 and EDC 20.98 Recently, a series of
reagents based on DMTMM 101 was developed by Kaminski
(Scheme 10).99 N-Triazinylammonium salts were synthesised
using different tertiary bases and the derivative incorporating
DABCO proved to give the best yield. However a full study
was carried out on the N-methylmorpholine derivative 102,
because of its lower production cost. The reagent proved to be
particularly efficient with high yields and low epimerisation
levels. For the synthesis of the 65–74 segment of ACP, each
coupling went faster (15 min.) than with TBTU 30b (45 min)
or HATU 28a (30 min) and gave better purities (84%) than
TBTU 30b (69%).99 Sulfonates of N-triazinylammonium salts
were also synthesised, but a complete evaluation of these
reagents was not reported.100 The reagents were further
optimised by replacing the methoxy groups by benzyloxy
groups (Fig. 16).101
Remarkably, reagents such as triazine 103 proved to be
stable in DMF with only 2.5% decomposition after 48 h.
Comparison between the parent methoxy compounds (e.g. 97)
and the benzyloxy derivatives (e.g. 103) showed that the later
were more efficient for the synthesis of the 65–74 segment
of ACP.
4.3 Pentafluorophenol (HOPfp)-based coupling reagents
(Fig. 17)
These types of reagents are based on the traditional penta-
fluorophenol leaving group and the generation of active esters.
They usually require the addition of HOAt as the level of
epimerisation is quite high: when coupling Z-Phe-Val-OH to
H-Pro-NH2, 33.7% of the LDL isomer was observed in solution
phase when using HPyOPfp 104a, while epimerisation
dropped to 1.7% when adding HOAt to the reaction mixture.
The use of a thiophenol-analogue, HPySPfp 104b did not
change the outcome of the coupling reactions.66 Like most
reagents based on HOAt/HOBt, these reagents are not ideal
for solution-phase chemistry as the use of an additive means
that this has to be removed from the reaction mixture after
coupling.
Li described a pentafluorophenyl immonium type reagent
FOMP 105,56 but this reagent was not as efficient as the other
immonium type reagents, based on HOBt/HOAt.
A reagent, PFNB 106, was reported by Pudhom, but
Boc-Gly-OH reacted slowly and incompletely and it was necessary
to add HOBt to get good conversion.102 In order to synthesise
thioamides, Hoeg-Jensen synthesised PyPOP 107, but this
reagent was not as efficient as PyNOP 53 or PyFOP 54.54
Other reagents include FDPP 108, which gave lower epimer-
isation levels than HBTU 28b, BOP 51b and DCC 5.103
Recently, HDMPfp 109 was synthesised by El-Faham but
the reagent proved to be outperformed by HATU 28a.50
Fig. 15 Structure of EEDQ and IIDQ.
Table 4 Comparison of EEDQ and IIDQ
Entry Amine AcidIIDQyield
EEDQyield
1 4-tert-Butylaniline
Phenylaceticacid
96 94
2 Benzylamine Phenylaceticacid
91 87
3 Morpholine Phenylaceticacid
38 32
4 4-tert-Butylaniline
Benzoic acid 88 85
5 Benzylamine Benzoic acid 85 666 Morpholine Benzoic acid 50 41
Average 76 67
Scheme 10 Exchange of counter anion on DMTMM 101.
Fig. 16 Structure of dibenzyloxytriazine 103.
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4.4 Reagents based on 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-
benzotriazine (HODhbt)
HODhbt was first mentioned in 1970 by Koenig who investi-
gated over 30 N-hydroxy compounds as additives for peptide
synthesis.25 HOBt gave excellent results but HODhbt proved
to be generally superior. However Koenig pointed out that the
potential of HODhbt is limited due to inherent side reactions,
in particular the formation of an azido-benzoyl derivative 110
(Fig. 18).
Knorr proposed the generation of a HODhbt based
coupling reagent, synthesising TDBTU 111 (Fig. 19).40
Although TDBTU 111 gave little epimerisation, its use was
recommended only in critical cases because of the risk of side
reactions. Indeed, ring opening of the 3,4-dihydro-4-oxo-1,2,3-
benzotriazine ring can occur to form 110, which can then react
with amines. Another reagent, HDTU 112b, where the
counter ion of TDBTU 111 was replaced by hexafluoro-
phosphate had similar efficiency to TBTU 30b.104 The
disadvantage of HDTU 112b has ever being its poor stability
in DMF compared to classic reagents such as HATU 28a as
after 5 h HDTU 112b had totally decomposed compared to
less than 1% for HATU 28a.37
Fig. 17 Coupling reagents based on pentafluorophenol.
Fig. 18 Side-product formed when using HODhbt as additive.
Fig. 19 Coupling reagents based on HODhbt.
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Carpino compared some organophosphorus reagents to
commonly used coupling reagents,65 and showed that
DpopODhbt 113 was comparable to HATU 28a in terms of
reaction times for the formation of the active ester of Z-Aib-OH
(o2 min) but DepODhbt 114 (also named DEPBT by
Ye105,106) was not as efficient (7–8 min). Similarly DOPBT
115 was poorer than DepODhbt 114.107 Another reagent,
DtpODhbt 116 gave more epimerisation (4.3% of LDL isomer)
than DepODhbt 114 (3.5%) but less than HATU 28a
(5.0%) when carrying out the coupling of Z-Phe-Val-OH
and H-Pro-NH2. The synthesis of the ACP decapeptide
(H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-NH2) was used
to show that DepODhbt 114 gave poor results (o1% yield)
compared to HATU 28a (85%).
Li also based immonium type reagents on HODhbt, but
DOMP 117 showed very poor results for the coupling between
Z-Gly-Phe-OH and H-Val-OMe with only 5.6% yield after 2 h
compared to 95% for BDMP for example.56 PyDOP 118a was
targeted for the synthesis of thioamides, but proved to be
surpassed by PyNOP 53 or PyFOP 54.54
More recently, Carpino developed coupling reagents based
on aza-analogues of HODhbt,65 and successfully synthesised
HDATU 112a, PyDAOP 118b, HDADU 119, HDAPyU
120a, and HDPyU 120b. As expected, derivatives of
HODAhbt were more reactive than their HODhbt analogue.
Thus, HDATU 112a gave better results than HDTU 112b, but
was still less reactive than HATU 28a. Moreover, results were
more random for segment coupling as they depended on the
system studied. However, in many cases, HDATU 112a
proved to be better than HATU 28a for the solid-phase
synthesis of ACP.
Itoh developed sulfonate reagents based on HODhbt.67 The
two reagents synthesised, SMDOP 121 and SPDOP 122 were
however not as efficient as the other sulfonate reagents that
this group synthesised, such as HCSCP 70.
Overall, reagents based on 3,4-dihydro-3-hydroxy-4-oxo-
1,2,3-benzotriazine (HODhbt) do not appear to be more
efficient that classical reagents like DIC 13. Moreover, a
critical issue regarding the safety of these materials has to be
addressed due to the presence of the azide moiety.
4.5 Reagents based on 2-hydroxysuccinimide (HOSu) and
2-(5-norbornene–2,3-dicarboximide) (HONB) (Fig. 20)
Only a few reagents incorporating the hydroxysuccinimide
leaving group have been synthesised. Knorr developed TSTU
123a and its norbornene–dicarboximide analogue TNTU 124,
which showed high epimerisation levels without the use of
additives.40 Gruber reported HSTU (also called SbTMU)
123b, but the reagent was not studied in detail as it
was directly used for the preparation of thiol-reactive Cy5
derivatives.108
Other examples are SOMP56 125 and SOMI57 126 developed
by Li, and similar other immonium type reagents, but they
gave poor results.
Phosphate-based succinimide coupling reagents such as
NDPP109 127 and SDPP110 128 have also been developed.
The use of ENDPP 129 proved to be a better method than the
isobutylchloroformate method because it could be performed
at room temperature, but no other comparison was reported.
Similarly, SDPP 128 was only reported as being a ‘‘more
convenient method’’ to use than DCC 5.
El-Faham reported the use of HDMS 130, which was based
on a morpholino uronium salt.50 The reagent proved to be less
efficient than the HOAt/HOBt based analogues HDMA 46a
and HDMB 46b.
4.6 Phosphorus-type reagents (not based on HOAt, HOBt,
–OPfP, –OSu, and –ODhbt) (Fig. 21)
PyTOP 131 was developed by Hoeg-Jensen for the formation
of thioamides but the reagent gave poorer selectivities than
PyNOP 53 or PyFOP 54.54
The possibility of using DPP-Cl 132 was first investigated
with success by Jackson,111 who claimed that NMR proved
that no epimerisation was observed,112 although this result is
quite surprising, as epimerisation is usually high when acid
chlorides are generated.
Other derivatives have also been synthesised and include
the azide analogue DPPA 133a,113 and cyano analogue
DECP 134, which gave good coupling yields but with many
side-reactions via the cyanide.114 Dpop-Cl 133b was also tested
but poor results were observed without the use of an additive.65
Similarly DEPC115 135a and DEPB116 135b typically give side
reactions due to the release of the reactive halogen atom.
Reagents based on the same principle, Cpt-Cl117 136,
MPTA118 137a, Mpt-Cl119 137b, MPTO118 138, and
BMP-Cl120 139, appeared overall to have similar efficiencies
to reagents such as DPP-Cl 132.
Fig. 20 Coupling reagents based on HOSu and HONB.
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Fig. 21 Other phosphorus-based reagents.
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Another coupling reagent TFMS-DEP 140 was produced by
activating diethylphosphate with trifluoromethanesulfonalide.121
Using 1.2 equiv. of coupling reagent, hindered tert-butylamine
was coupled in 89% yield to acetic acid. Other examples
showed goods yields, typically over 80% yield, including a
secondary amine (N-methylbenzylamine) and two anilines
(N-methylaniline and aniline). Application for peptide synthesis
was studied by carrying out Young’s test, which showed 2%
epimerisation. Also, the difficult synthesis of Z-Aib-Aib-OMe
proved to be successful affording the product in a satisfactory
70% yield.
A wide range of phosphorus-based coupling reagents
141–153 were investigated by Mukaiyama.122 Using Young’s
test as model reaction, it was concluded that the bis(nitrophenyl)
phenylphosphonates 149 and 150 gave the best results. Further
studies, using this time phosphinic esters 154–158 showed
that (5-nitropyridyl)diphenylphosphinate 154 was an efficient
coupling reagent, giving 92% of the expected dipeptide in
Young’s test, with less than 2% epimerisation.123
DEBP124159 and DPOOP125
160 have been proposed as
coupling reagents, but for both reagents, examples were
limited to a few dipeptides and were not compared to any
classical methods. T3P 161 was claimed to be more efficient
than HAPyU 31 for head-to-tail cyclisation of hindered
peptides.126 However, the use of T3P may be limited as yields
were lower and epimerisation higher than HAPyU when
segment coupling studies were carried out.
Other reagents include FDMP 162, which gave poor results
(2% yield compared to 84% yield for BEMT when coupling
Z-Gly-Phe-OH to H-Val-OMe),57 BIODPP 163, which gave
amides in good yields but was not compared to any other
coupling reagent,127 and DEPBO 164 and DOPBO 165, which
proved to be not as efficient as DepODhbt 114.107 PyDPP 166
was reported as giving low epimerisation rates, but was not
compared to other coupling reagents.128
Kokare reported three new reagents 166–169 based on
phosphate derivatives of 1-hydroxy-2-phenylbenzimidazole.129
The reagents gave in most cases similar results and yields over a
wide range of substrates (e.g. 4-nitrobenzoic acid, cinnamic
acid, anisic acid, piperidine, tert-butylamine) were excellent.
However, one can wonder at the purity of the isolated products.
The synthesis of the three reagents were reported (63–71% yields),
but when used for amide bond formation, the reagents were
generated in situ through the reaction of 2-phenylbenzimidazole
with a chlorophosphate or phosphinic chloride. The acid
and then amine were added to this mixture, and side-
reactions were thus likely to occur. Kokare also used the
diethylphosphate derivative 170 as a coupling reagent for the
synthesis of O-alkyl hydroxamic acids (Scheme 11).130 Yields
were excellent for the 12 amides synthesised but comparison
with other coupling reagents was not carried out.
4.7 Miscellaneous reagents
CPMA 171, a reagent based on a chloroimmonium salt
(Fig. 22), mediated the esterification of carboxylic acids,131
and in terms of amide bond formation, the reagent performed
well (complete conversion) but only two examples were
reported.
2-Mercaptopyridone-1-oxide 172 was used as a starting
material to generate a cheaper and new type of uronium
coupling reagent TOTT 173 and HOTT 174 (Scheme 12).132
Both reagents gave better results that DCIH 78b or PyBrop
79b and were comparable to HATU 28a, and the dipeptide
Z-MeVal-Aib-OMe was obtained in 80% yield (89% for
HATU 28a). The epimerisation level was evaluated via
Young’s test and the use of TOTT 173 resulted in only 3.7%
epimerisation compared to BOP 51b (20%), PyBOP 52b
(15%), or HATU 28a (20%). TOTT 173 and HOTT 174 have
also been successfully used to synthesise primary amides from
carboxylic acids and ammonium chloride.133
Najera synthesised two analogues of HOTT/TOTT, HODT
175 and TODT 176 (Fig. 23).134 These two reagents gave
higher yields in solid phase peptide synthesis, but associated
with more epimerisation.
A reagent similar to the ones based on 2-mercaptopyridine
oxide was proposed by Knorr but TPTU 177 (Fig. 24), based
on 2-hydroxypyridine-N-oxide, gave high epimerisation level
when used without an additive.40
The possibility of using a 2-pyridinone based reagent,
DPTC 178 (Fig. 25), for amide synthesis was investigated by
Shiina.135 Carboxylic acids were activated as 2-pyridyl esters
using DPTC 178 and a catalytic amount of DMAP. However,
a long pre-activation time was required (over 25 min) to limit
the formation of an isothiocyanate specie (and probably a
thiourea) upon addition of an amine. Thus the application of
DPTC 178 is limited although simple amides can be obtained
in good yield at room temperature. More hindered substrates
imply carrying out the synthesis at higher temperature.
An original coupling reagent based on the rearrangement of
carboxylic–sulfonic mixed anhydrides has been reported. Sub-
stituted O-hydroxybenzenesulfonyl chlorides 179 were used as
condensation reagents via the mechanism suggested in
Scheme 13.136 Using this method various peptides were
obtained in good yields. The epimerisation level was assessed
through optical purity, but no comparison was made with any
common coupling reagent. Itoh investigated the possibility of
using sulfonate-based coupling reagents, and developed
2-methanesulfonyloximino-2-cyanoacetate 180 (Fig. 26),
which proved however to be outperformed by HCSCP 69.67
Scheme 11 Synthesis of O-alkyl hydroxamic acids. Fig. 22 Structure of CPMA.
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A related reagent, also based on a cyanoacetate moiety,
TOTU 181 was reported by Konig.137
Carbonyl-diimidazole (CDI 182) has been used to generate
amide bonds.138 Interestingly, Sharma showed that CDI 182
Scheme 12 Synthesis of HOTT and TOTT from 2-mercaptopyridone.
Fig. 23 Structure of HODT and TODT.
Fig. 24 Structure of TPTU.
Fig. 25 Structure of DTPC.
Scheme 13 Mechanism of the coupling reagents using substituted O-hydroxybenzenesulfonyl chlorides.
Fig. 26 Structure of other miscellaneous reagents.
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could be used to couple unprotected amino acids to amines in
water.139 The strategy however offers limited applicability as
only primary amines were successfully coupled, while yields
were moderate.
More recently, Saha proposed the use of an analogue,
CBMIT 183.140 He obtained good yields and low epimerisation
but these were not evaluated on standard tests and are
therefore difficult to compare to classical reagents.
DPTF 184 was reported by Ito as a dehydrating reagent.141
Its mechanism of action follows the active ester pathway
to generate amides in good yields (Scheme 14). However
hindered building blocks were not evaluated. One of the main
advantages of DPTF 184 is its ability to activate a carboxylic
acid in aqueous media.
In order to avoid the use of expensive reagents, Campagne
suggested the use of ethyl propiolate 185 as coupling reagent,
as described in Scheme 15.142 Although being original, this
route required a long pre-activation time (12 h) and the use of
an additive (sodium bisulfite) was necessary to give good
yields. Moreover, yields were typically lower than standard
coupling reagents such as PyBOP 52b.
Recently, diphenyl phosphite (DPP 186),143 and tetrakis-
(pyridine-2-yloxy)silane 187,144 have been used to synthesise
amides. DPP 186 forms a phosphonic-carboxylic mixed
anhydride, while tetrakis(pyridine-2-yloxy)silane gives silyl
esters 188 (Scheme 16). These reagents afforded amides in
good yields but were not compared to other coupling reagents.
Phenylsilane PhSiH3 189 has been used in amide library
formation.145 The reagent was tested on seven carboxylic acids
and 11 amines. Although amides were sometimes obtained in
good yield, it was necessary to use reverse phase HPLC to purify
the products, making the phenylsilane method unattractive for
library generation. In addition, anilines and some secondary
amines failed to couple with this reagent resulting in poor scope.
5. Other methods of N-acylation
5.1 Mixed anhydrides
The formation of mixed anhydrides is a classic method of
amide bond formation. It is important to note that many
mixed anhydrides can be generated using some of the coupling
reagents reported so far in this review. The mixed anhydride
method was first reported by Vaughan,146 who tested many
acid chloride derivatives and concluded that the success of the
amide-bond formation was governed by steric and inductive
effects. Isovaleryl chloride proved to give the best results.
However, as reported by many research groups, this method
has a tendency to generate symmetrical anhydrides by reaction
of a second carboxylic acid molecule on the mixed anhydride
(Scheme 17). In addition regioselectivity is a major issue, as the
amine can potentially react at either carbonyl group although
this can be biased by using a bulky acid group. These
drawbacks can sometimes be minimised by carrying out the
coupling reactions at low temperature.
5.2 Chloroformates
The use of chloroformates for amide-bond formation was
first reported by Vaughan,147 and was based on the mixed
anhydride method. In the presence of a base, the reaction
between a carboxylate and a chloroformate yields a mixed
carbonic anhydride, which reacts quickly with amines to form
amides. Vaughan’s study highlighted slightly better results
when using sec-butylchloroformate compared to isobutylchloro-
formate.148 The method was ‘‘reinvestigated’’ by Anderson,149
who tested several different chloroformates, and whose
conclusions suggested that isobutylchloroformate was the
most efficient reagent.
Scheme 14 Suggested mechanism of DPTF.141
Scheme 15 Activation process when using ethyl propiolate as coupling reagent.
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5.3 Direct preparation of active esters
The direct formation of active esters has often attracted a lot
of attention due to the stability of many of them, which allows
storage. Many example of active esters have therefore been
reported and include –O-succinimides,150 –OBt and derivatives,24
p-nitrophenol,151 –OPfP,152 –ODhbt,153 and PTOC.154 As this
review focuses directly on coupling reagents, this useful
method of amide-bond formation will not be discussed
herein, but the reader is referred to Montalbetti’s review for
further details.13
5.4 Newer approaches to amide bond formation
Several alternatives to the use of coupling reagents have been
reported. These interesting new methods were reviewed by
Bode,155 and include the so-called native chemical ligation and
the Staudinger ligation (Scheme 18). Recently, Milstein
reported another approach based on the ligation of amines
to alcohols using a ruthenium complex as catalyst.156
Molecular hydrogen was formed during the reaction and
amides were obtained in high yield.
6. Polymer-supported coupling reagents
6.1 Immobilised carbodiimides
Only a few polymer-supported coupling reagents are available,
probably because coupling reagents are mainly used in peptide
synthesis, which is usually carried out on solid phase, the
coupling reagent being in solution. Nevertheless, DCC 5,157
DIC 13,158 and EDC159 20 have been successfully immobilised
and applied to the synthesis of amides.160 However these
carbodiimides maintain the same drawbacks as their
solution-phase equivalents, in particular in terms of epimerisation
in the absence of an additive. Furthermore, one can wonder
at the interest of PS-EDC 190 (Fig. 27) in comparison to
PS-DCC 191 as EDC 20 was originally designed and synthesised
to be water soluble. Having the ‘‘extractable’’ moiety on a
polystyrene support appears to be odd, especially as the
ionic part of EDC 20 in solution-phase has proven to be
counterproductive regarding the coupling reaction rate
compared to DIC 13.34 A polyhexamethylene-carbodiimide
has also been reported.161
Charette ‘‘attached’’ carbodiimides to tetraarylphosphonium
salts as a means of ‘‘tagging’’ the reagent.162 Reaction
was carried out in solution phase, before precipitation of the
salt with apolar solvents. Several carbodiimides derivatives
192 were synthesised (Fig. 28), and the ethyl and isopropyl
derivatives based on a hexafluorophosphate salt were the most
efficient, both in terms of yields and purities.
6.2 Immobilised additives and reagents based on HOBt
Some coupling reagents in solution can in rare cases be
extracted after reaction (e.g. EDC 20). However, the use of
an additive is often required to limit epimerisation, and this
additive has also to be separated from the reaction mixture.
Therefore polymer-supported HOBt has been reported in
different guises.163,164 PS-HOBt 193 has also been used as a
core for synthesising supported reagents for the preparation of
N-hydroxysuccinimide active esters.165
The idea of using PS-HOBt 193 to form an immobilised
HOBt-based coupling reagent was first exploited by Chinchilla,
who synthesised polymer-supported TBTU 194.166 This idea
was also applied by Filip for the synthesis of polymer-
supported BOP 195.167 These reagents offer however the same
Scheme 16 Mechanism proposed by Tozawa for tetrakis(pyridine-
2-yloxy)silane.
Scheme 17 Disproportionation issue with the mixed anhydride
method.
Scheme 18 Examples of newer methodologies for amide bond formation.
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drawbacks as TBTU 30b and BOP 51b in solution, while the
structure of the reagent means that part of it will end up in
solution after the coupling, clearly an undesirable occurrence
for a supported reagent.
6.3 Other immobilised reagents
Triazine-based coupling reagents have been widely used in
solution-phase. In 1999, Taddei reported polymer-supported
chlorotriazine 196.168 Although amides were synthesised in
moderate to good yield using this reagent, the 1H NMR of the
crude compounds revealed the presence of 5 to 10% of
by-products. Hioki used another strategy to obtain polymeric
triazine-type reagents.169 Using a norbornene-derivatised
triazine, they synthesised via ROMP an immobilised mono-
methoxychlorotriazine, which was tested on anilines and
primary amines. Yields were good (nine examples, 80–98%),
but no secondary amine was tested while the reagent was not
compared to other classical amide bond formation methods.
PS-DMC 197, a supported equivalent of DMC 89, was
reported by Ishikawa.170 Yields over five examples were
slightly lower for the polymer-supported version of the
reagent, and the examples provided did no allow a full display
of the scope and limitations of the reagent.
Chinchilla developed some reagents based on polymeric
succinimides such as P-TSTU 198 and P-HSTU 199,171 and
200 (Fig. 27).172 The results were good for classic amino acids
but the yields were moderate to low when coupling hindered
amino acids. Globally these reagents did not really add any
benefit to the range of coupling reagents available, and, like
PS-TBTU 194 and PS-BOP 195, part of the reagent ended up
in solution.
More recently, Convers reported an immobilised Mukaiyama
reagent 201.173 However, Crosignani investigated this new
reagent and concluded that the synthesis was poorly reprodu-
cible, and developed another route.174 This reagent 202
appeared to work very efficiently for the synthesis of esters
and amides including hindered substrates, secondary amines
and anilines.174,175
Polymer-supported IIDQ 203 is an immobilised version of
the solution-phase IIDQ 100 reagent.97,176 It was synthesised
in three steps from Merrifield resin and 6-hydroquinoline to
provide a high loading reagent (41.68 mmol/g). The main
Fig. 27 Structure of polymer-supported reagents.
Fig. 28 Tetraarylphosphonium-supported carbodiimides.
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advantages of PS-IIDQ 203 are that no base is required during
coupling, while the order of addition of amine, carboxylic acid
and reagent do not influence the outcome of the reaction
(Scheme 19).
This reagent was compared to other classically used and
commercially available coupling reagents such as Polymer-
supported EDC 190 and DCC 191, as well as HATU 28a.
Interestingly, PS-IIDQ 203 performed better than any of these
reagents on a set of three amines and three carboxylic acids,
including anilines and bulky substrates (Table 5). Furthermore,
PS-IIDQ 203 was evaluated on 9 amines and 5 carboxylic
acids and gave an average yield of 73%. Epimerisation was
low as Anteuni’s test did not reveal any trace of the diastereo-
isomer by NMR. PS-IIDQ 203 was stable under standard
laboratory storage conditions and it was shown that the
reagent could be advantageously recycled after any coupling
reaction. Thus PS-IIDQ 203 appears to be a very versatile
coupling reagent for the parallel synthesis of amides.
Very recently, Kakarla duplicated these studies to make
PS-EEDQ 204.177 It was obtained using identical conditions
for the transformation of PS-Quinoline into PS-EEDQ
204, the only variation being the use of a Wang resin. However
the loading of the so-called ‘‘high-loading’’ PS-EEDQ 204
was erroneous (starting from a 1.7 mmol/g Wang resin, the
maximum physical loading of PS-EEDQ 204 would be
1.19 mmol/g assuming total conversion during synthesis,
while the authors claimed 1.36 mmol/g loading), while a Wang
linker was clearly of no use. When looking at the efficiency
of EEDQ 99 and IIDQ 100 (Table 4),97 the choice
appears evident.
7. Conclusion on available coupling reagents
Although hundreds of coupling reagents have been reported,
conclusions on their efficiency are in fact quick and simple.
Most of these reagents are simply not efficient for a broad
range of amide bond formation. Some reagents do perform
well in general, but differences are typically small. Solid-phase
peptide chemists may find useful reagents which display fast
kinetics for coupling as the synthesis of long peptides has
ideally to be rapid. However, for the general organic chemist,
simple reagents are often the most appropriate allowing
coupling reagents to be used on a large selection of substrates
with varying reactivities.
This summary can be illustrated by the comparison of
coupling reagents carried out by Hachman.68 Very few
comparisons of reagents have been published and the work by
Hachman displayed the importance of a comparison system.
Hachman compared classical reagents such as phosphonium
salts, uronium salts, reagents generating acid halides and
carbodiimides. During the synthesis of decapeptides, HBTU
28b was the ‘‘fastest’’ reagent after 2 min while almost none of
the expected amide was formed by DIC after this time.
However, after 8 min, DIC 13 was comparable to HBTU
28b. In addition very few side-reactions were observed with
DIC 13 (in particular deletion) compared to BOP 51b or
HATU 28a. This demonstrated that a simple reagent like
DIC 13 (using HOBt as additive) performs well in many cases,
and a compromise of speed/purity/by-products needs to be
sought.
An important point is the way new coupling reagents are
reported. As stated and demonstrated by Hachman: ‘‘the use
of only one model sequence for evaluation of synthetic
reagents [. . .] can be misleading.’’ As such, unless new reagents
are systematically tested against commonly considered ‘‘top
coupling reagents’’, such as HATU 28a, and traditional
methods such as DIC/HOBt, it is likely that most new
coupling reagents will have an application limited to the
original publication by their authors.
Overall, keeping in mind all possible issues (side-reactions),
HATU 28a and HBTU 28b offer generally excellent reactivity.
Scheme 19 Activation process when using PS-IIDQ.
Table 5 Comparison of the yields and purities obtained over threeamines (4-tert-butylaniline, benzylamine, H-PhG-OMe) and threecarboxylic acids (Boc-Aib-OH, phenylacetic acid, benzoic acid)
Entry Coupling reagent Average yield (%) Average purity (%)
1 PS-IIDQ 72 1002 HATU 55 983 PS-EDC 41 964 PS-DCC 26 97
This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 606–631 | 625
If quick coupling times are required, HATU 28a probably
represents the reagent of choice, providing the substrates are
not hindered. Otherwise, the traditional method DCC 5 (or
DIC 13) /HOBt remains an excellent choice for many sub-
strates. One has nevertheless to keep in mind potential hazards
when using reagents based on 1H-benzotriazole due to the
potential explosive properties of HOBt.30,31
For difficult couplings (e.g. secondary amines), our experi-
ence tells us that PyBrop 79b is generally reliable.178 Triazines
can be an alternative for difficult coupling, although the most
reactive reagents tend to give side-products. However, the
recent developments by Kaminski are bringing new applica-
tions to this class of coupling reagents.
Finally, for library synthesis either the PS-Mukaiyama
reagent 202 or polymer-supported IIDQ 203 are clearly the
most suitable reagents,179 and their efficiency has been con-
firmed by many groups. These reagents have the advantage of
simplifying purification as the reagent is separated via simple
filtration after reaction.
In conclusion, selecting suitable coupling reagents could be
summarised by ‘‘keep it simple’’ as most reagents appear to be
merely fancy and costly alternatives. Finding a universal
coupling reagent remains elusive considering the wide portfolio
of potential substrates and it is generally wise to avoid
‘‘exotic’’ reagents and not be mislead by ‘‘fast’’ coupling
reagents. Efficiency is the key, with high conversions, low
levels of epimerisation and limited by-products all being
essential criteria.
List of abbreviations
General
ACP acyl carrier protein decapeptide 65–74
DABCO bicyclo[2,2,2]-1,4-diazaoctane
DCU dicyclohexylurea
DMAP 4-dimethylaminopyridine
DMPU dimethylpropyleneurea
HMPA hexamethylphosphoramide
LHRH Luteinising Hormone Releasing Hormone
NMM N-methylmorpholine
ROMP Ring Opening Metathesis Polymerisation
Coupling reagents and additives
ACTU (2-(6-chloro-1-H-benzotriazol-1-yl)-1,1,3,3-
tetramethylaminium) hexachloroantimonate
AOMP 5-(7-azabenzotriazol-1-yloxy)-3,4-dihydro-1-
methyl-2H-pyrrolium hexachloroantimonate
AOP (7-azabenzotriazol-1-yl)oxytris(dimethyl-
amino)phosphonium hexafluorophosphate
BBC benzotriazoloxy-bis(pyrrolidino)carbonium
hexafluorophosphate
BDDC bis[[4-(2,2-dimethyl-1,3-dioxolyl)]methyl]-
carbodiimide
BDMP 5-(1H-benzotriazol-1-yloxy)-3,4-dihydro-1-
methyl-2H-pyrrolium hexachloroantimonate
BDP benzotriazol-1-yl diethylphosphate
BEC N-tert-butyl-N0-ethylcarbodiimide
BEMT 2-bromo-3-ethyl-4-methylthiazolium
tetrafluoroborate
BEP 2-bromo-1-ethylpyridinium tetrafluoroborate
BEPH 2-bromo-1-ethylpyridinium hexachloroanti-
monate
4,5-B(HATU) N-[(dimethylamino)(3H-1,2,3-triazolo[4,5-c]-
isoquinolin-3-yloxy)-N-methylmethanaminium
hexafluorophosphate
5,6-B(HATU) 1-[bis(dimethylamino)methylene]-1H-1,2,3-
triazolo[4,5-b]quinolinium hexafluorophosphate-
3-oxide
BIODPP diphenyl benzo[d]isoxazol-3-ylphosphonate
BMC N-tert-butyl-N0-methylcarbodiimide
BMMP 1-(1-(1H-benzo[d][1,2,3]triazol-1-yloxy)ethyl-
idene)pyrrolidinium hexachloroantimonate
BMP-Cl N,N0-bismorpholinophosphonic chloride
BMTB 2-bromo-3-methyl-4-methylthiazolium
bromide
BOI 2-(benzotriazol-1-yl)oxy-1,3-dimethylimid-
azolidinium hexafluorophosphate
BOMI benzotriazol-1-yloxy-N,N-dimethylmethan-
iminium hexachloroantimonate
BOP benzotriazolyl-N-oxytrisdimethylaminophos-
phonium hexafluorophosphate
BOP-Cl N,N0-bis(2-oxo-3-oxazolidinyl)phosphinic
chloride
BPMP 1-(1H-benzotriazol-1-yloxy)phenylmethylene
pyrrolidinium hexachloroantimonate
BroP bromotris(dimethylamino)phosphonium
hexafluorophosphate
BTFFH bis(tetramethylene)fluoroformamidinium
hexafluorophosphate
CBDO 2-chlorobenzo[d][1,3]dioxol-1-ium hexachloro-
antimonate
CBMIT 1,10-carbonylbis(3-methylimidazolium) triflate
CDI carbonyldiimidazole
CDMS chlorodimethylsulfonium hexachloroantimonate
CDMT 2-chloro-4,6-dimethoxy-1,3,5-triazine
CDTP 2-chloro-1,3-dimethyl-3,4,5,6-tetrahydro-
pyrimidin-1-ium perchlorate
CIP 2-chloro-1,3-dimethylimidazolidinium
hexafluorophosphate
CloP chlorotris(dimethylamino)phosphonium
hexafluorophosphate
CMMM chloro(4-morpholino)methylene
morpholinium hexafluorophosphate
CPMA (chlorophenylthiomethylene)dimethyl-
ammonium chloride
CPDT 2-chloro-5-phenyl-1,3-dithiol-1-ium
hexachloroantimonate
Cpt-Cl 1-oxo-chlorophospholane
DAST diethylaminosulfur trifluoride
DCC dicyclohexylcarbodiimide
DCIH 1,3-dimethyl-2-chloro-4,5-dihydro-1H-
imidazolium hexafluorophosphate
DCMT 2,4-dichloro-6-methoxy-1,3,5-triazine
DEBP diethyl-2-(3-oxo-2,3-dihydro-1,2-benziso-
sulfonazolyl)phosphonate
626 | Chem. Soc. Rev., 2009, 38, 606–631 This journal is �c The Royal Society of Chemistry 2009
DEFFH 1,2-diethyl-3,3-tetramethylenefluoroform-
amidinium hexafluorophosphate
DECP diethylcyanophosphonate
DEPC diethyl phosphorochloridate
DEPB diethyl phosphorobromidate
DEPBO N-diethoxyphosphorylbenzoxazolone
DEPBT 3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-
4(3H)-one
DepOAt 3H-[1,2,3]triazolo[4,5-b]pyridin-3-yldiethyl
phosphate
DepOBt diethoxyphosphinyloxybenzotriazole
DepODhbt diethyl 4-oxobenzo[d][1,2,3]triazin-3(4H)-yl
phosphate
DFIH 1,3-dimethyl-2-fluoro-4,5-dihydro-1H-imid-
azolium hexafluorophosphate
DIC diisopropylcarbodiimide
DMC 2-chloro-1,3-dimethylimidazolinium chloride
DMCH N-(chloro(morpholino)methylene)-N-methyl-
methanaminium hexafluorophosphate
DMFFH 1,2-dimethyl-3,3-tetramethylenefluoroform-
amidinium hexafluorophosphate
DMFH N-(fluoro(morpholino)methylene)-N-methyl-
methanaminium hexafluorophosphate
DmppOAt 1-(2,8-dimethylphenoxaphosphinyloxy)-7-
azabenzotriazole
DMTMM 4-(4,6-dimethoxy[1,3,5]triazin-2-yl)-4-methyl-
morpholinium chloride
DOMP 5-(30,40-dihydro-40-oxo-10,20,30-benzotriazin-
30-yloxy)-3,4-dihydro-1-methyl 2H-pyrrolium
hexachloroantimonate
DOPBO N-(2-oxo-1,2,3-dioxaphosphorinanyl)benz-
oxazolone
DOPBT 3-[O-(2-oxo-1,2,3-dioxaphosphorinanyl)oxy]-
1,2,3-benzotriazin-4(3H)-one
DPOOP diphenyl-2-oxo-3-oxazolinylphosphonate
Dpop-Cl diphenyl phosphorochloridate
DpopOAt 1-(diphenoxyphosphoryloxy)-7-azabenzo-
triazole
DpopOBt 1-(diphenoxyphosphoryloxy)benzotriazole
DpopODhbt 3-(diphenoxyphosphinyloxy)-3,4-dihydro-4-
oxo-1,2,3-benzotriazene
DPP diphenylphosphite
DPPA diphenylphosphoryl azide
Dpp-Cl diphenylphosphinic chloride
DPTC O,O0-di(2-pyridyl)thiocarbonate
DPTF 2,2-dichloro-5-(2-phenylethyl)-4-(trimethylsilyl)-
3-furanone
DtpOAt 1-[di(O-tolyl)phosphinyloxy]-7-azabenzotriazole
DtpOBt 1-[di(O-tolyl)phosphinyloxy]benzotriazole
DtpODhbt 3-di(O-tolyl)phosphinyloxy]-3,4-dihydro-4-
oxo-1,2,3-benzotriazine
EDC 1-ethyl-3-(3-dimethylaminopropyl)carbodi-
imide
EEDQ N-ethoxycarbonyl-2-ethoxy-1,2-dihydro-
quinoline
ENDPP phosphoric acid 3,5-dioxo-10-oxa-4-azatri-
cyclo[5.2.1.02,6]dec-8-en-4-yl ester diphenyl ester
FDMP 3,5-bis(trifluoromethyl)phenyl
diphenylphosphinate
FDPP pentafluorophenyl diphenyl phosphinate
FEP 2-fluoro-1-ethylpyridinium tetrafluoroborate
FEPH 2-fluoro-1-ethylpyridinium hexachloroanti-
monate
FOMP 5-(pentafluorophenyloxy)-3,4-dihydro-1-methyl-
2H-pyrrolium hexachloroantimonate
HAE2PipU O-(1H-1,2,3-triazolo[4,5-b]pyridin-1-yl)-1,1-
diethyl-3,3-pentamethyleneuronium
hexafluorophosphate
HAE2PyU O-(1H-1,2,3-triazolo[4,5-b]pyridin-1-yl)-1,1-
diethyl-3,3-tetramethyleneuronium hexafluoro-
phosphate
HAMDU O-(7-azabenzotriazol-1-yl)-1,3-dimethyl-1,3-
dimethyleneuronium hexafluorophosphateHAM2PipU O-(1H-1,2,3-triazolo[4,5-b]pyridin-1-yl)-1,1-
dimethyl-3,3-pentamethyleneuronium
hexafluorophosphate
HAM2PyU O-(1H-1,2,3-triazolo[4,5-b]pyridin-1-yl)-1,1-
dimethyl-3,3-tetramethyleneuronium
hexafluorophosphate
HAMTU O-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(penta-
methylene)uronium hexafluorophosphate
HAPipU O-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(penta-
methylene)uronium hexafluorophosphate
HAPyTU S-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(tetra-
methylene)thiouronium hexafluorophosphate
HAPyU 1-(1-pyrrolidinyl-1H-1,2,3-triazolo[4,5-b]pyridin-
1-ylmethylene)pyrrolidinium hexafluorophos-
phate N-oxide
HATeU O-(1H-1,2,3-triazolo[4,5-b]pyridin-1-yl)-
1,1,3,3-tetraethyluronium
hexafluorophosphate
HATU O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetra-
methyluronium hexafluorophosphateHBE2PipU
O-(1H-benzotriazol-1-yl)-1,1-diethyl-3,3-penta-
methyleneuronium hexafluorophosphate
HBE2PyU O-(1H-benzotriazol-1-yl)-1,1-diethyl-3,3-tetra-
methyleneuronium hexafluorophosphate
HBMDU O-(benzotriazol-l-yl)-l,3-dimethyl-l,3-di-
methyleneuronium hexafluorophosphate
HBMP 1H-benzo[d][1,2,3]triazol-1-ylmethanesulfonate
HBM2PipU O-(1H-benzotriazol-1-yl)-1,1-dimethyl-3,3-
pentamethyleneuronium hexafluorophosphate
HBM2PyU O-(1H-benzotriazol-1-yl)-1,1-dimethyl-3,3-
tetramethyleneuronium hexafluorophosphate
HBPipU O-(benzotriazol-1-yl)-1,1,3,3-bis(pentamethylene)-
uronium hexafluorophosphate
HBSP 1H-benzo[d][1,2,3]triazol-1-ylbenzenesulfonate
HBTeU O-(1H-benzotriazol-1-yl)-1,1,3,3-tetraethyl-
uronium hexafluorophosphate
HBTU O-(benzotriazol-1-yl)-1,1,3,3-tetramethyl-
uronium hexafluorophosphate
HCTU (2-(6-chloro-1-H-benzotriazol-1-yl)-1,1,3,3-
tetramethylaminium) hexafluorophosphate
HCSCP 6-chloro-1H-benzo[d][1,2,3]triazol-1-yl-4-
chlorobenzenesulfonate
HCSP 6-chloro-1H-benzo[d][1,2,3]triazol-1-ylbenzene-
sulfonate
This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 606–631 | 627
HDATU (bis(dimethylamino)methyl)(4-oxopyrido[3,2-d]-
[1,2,3]triazin-3(4H)-yl)oxonium
hexafluorophosphate
HDADU (bis(dimethylamino)methyl)(4-oxopyrido[3,2-d]-
pyrimidin-3(4H)-yl)oxonium
hexafluorophosphate
HDAPyU 1-((4-oxopyrido[3,2-d][1,2,3]triazin-3(4H)-yloxy)-
(pyrrolidin-1-yl)methylene)pyrrolidinium
hexafluorophosphate
HDMA 1-((dimethylamino)(morpholino)methylene)-
1H-[1,2,3]triazolo[4,5-b]pyridinium hexafluoro-
phosphate 3-oxide
4-HDMA 3-((dimethylamino)(morpholino)methylene)-
1H-[1,2,3]triazolo[4,5-b]pyridinium hexafluoro-
phosphate 1-oxide
HDMB 1-((dimethylamino)(morpholino)methylene)-
1H-benzotriazolium hexafluorophosphate
3-oxide
HDMCB 6-chloro-1-((dimethylamino)(morpholino)-
methylene)-1H-benzotriazolium
hexafluorophosphate 3-oxide
HDMFB 6-trifluoromethyl-1-((dimethylamino)-
(morpholino)methylene)-1H-benzotriazolium
hexafluorophosphate 3-oxide
HDMPfp 1-((dimethyamino)(morpholino))oxypenta-
fluorophenyl metheniminium hexafluoro-
phosphate
HDMS 1-((dimethyamino)(morpholino))oxypyrrolidine-
2,5-dione methanaminium hexafluorophosphate
HDPyU 1-((4-oxobenzo[d][1,2,3]triazin-3(4H)-yloxy)-
(pyrrolidin-1-yl)methylene)pyrrolidinium
hexafluorophosphate
HDTMA 1-((dimethylamino)(thiomorpholino)methylene)-
1H-[1,2,3]triazolo[4,5-b]pyridinium hexafluoro-
phosphate 3-oxide
HDTMB 1-((dimethylamino)(thiomorpholino)methylene)-
1H-benzotriazolium hexafluorophosphate
3-oxide
HDTU O-(3,4-dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-
1,1,3,3-tetramethyluronium hexafluorophosphate
HOAt 1-hydroxy-7-azabenzotriazole
HOBt 1-hydroxy-1H-benzotriazole
HODhat 3-hydroxy-4-oxo-3,4-dihydro-5-azabenzo-
1,2,3-triazine
HODhbt 3,4-dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine
HODT S-(1-oxido-2-pyridinyl)-1,3-dimethyl-1,3-tri-
methylenethiouronium
HONB 2-(5-norbornene-2,3-dicarboximide)
HOPfp pentafluorophenol
HPyOPfp N,N,N0,N0-bis(tetramethylene)-O-pentafluoro-
phenyluronium hexafluorophosphate
HPySPfp 1-((perfluorophenylthio)(pyrrolidin-1-yl)-
methylene)pyrrolidinium hexafluorophosphate
HOSu N-hydroxysuccinimide
HOTT S-(1-oxido-2-pyridinyl)-1,1,3,3-tetra-
methylthiouronium hexafluorophosphate
HSTU O-(N-succimidyl)-N,N,N0,N0-bis(tetramethylene)-
uronium hexafluorophosphate
IDDQ N-isobutoxycarbonyl-2-isobutoxy-1,2-dihydro-
quinoline
MPTA dimethylphosphinothioyl azide
MPT-Cl dimethylphosphinothioyl chloride
MPTO 3-dimethylphosphinothioyl-2(3H)-oxazolone
NDPP norborn-5-ene-2,3-dicarboximidodiphenyl-
phosphate
NOP [(6-nitrobenzotriazol-1-yl)oxy]tris(dimethyl-
aminop)phosphonium hexafluorophosphate
PEC phenylethylcarbodiimide
PFNB perfluorophenyl 4-nitrobenzenesulfonate
PIC phenylisopropylcarbodiimide
PTOC pyridine-2-thione-N-oxycarbonyl
PyAOP [(7-azabenzotriazol-1-yl)oxy]tris(pyrrolidino)-
phosphonium hexafluorophosphate
PyBOP benzotriazol-1-yloxytri(pyrrolidino)-
phosphonium hexafluorophosphate
PyBroP bromotri(pyrrolidino)phosphonium
hexafluorophosphate
PyClock 6-chloro-1-hydroxybenzotriazol-1-yl-N-oxy-
tris(pyrrolidino)phosphonium
hexafluorophosphate
PyCloP chlorotri(pyrrolidino)phosphonium hexafluoro-
phosphate
PyClopP chlorobispyrrolidinophenylphosphonium
hexachloroantimonate
PyFloP fluorotri(pyrrolidino)phosphonium
hexafluorophosphate
PyClU chlorodipyrrolidinocarbenium
hexafluorophosphate
PyDAOP (4-oxopyrido[3,2-d][1,2,3]triazin-3(4H)-yloxy)-
tripyrrolidin-1-ylphosphonium
hexafluorophosphate
PyDOP [(3,4-dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-
oxy]tris(pyrrolidino)phosphonium
hexafluorophosphate
PyDPP diphenyl 2-oxopyridin-1(2H)-ylphosphonate
PyFOP [[6-(trifluoromethyl)benzotriazol-1-yl]oxy]tris-
(pyrrolidino)phosphonium hexafluorophosphate
PyNOP [(6-nitrobenzotriazol-1-yl)oxy]tris(pyrrolidino)-
phosphonium hexafluorophosphate
PyPOP (perfluorophenoxy)tripyrrolidin-1-ylphosphonium
PyTOP (pyridyl-2-thio)tris(pyrrolidino)-phosphonium
hexafluorophosphate
SbTMU O-(N-succimidyl)-N,N,N0,N0-bis-(tetramethylene)-
uronium hexafluorophosphate
SDPP 2,5-dioxopyrrolidin-1-yl diphenyl phosphate
SMDOP 4-oxobenzo[d][1,2,3]triazin-3(4H)-yl
methanesulfonate
SPDOP 4-oxobenzo[d][1,2,3]triazin-3(4H)-yl
benzenesulfonate
SOMI 5-(succinimidyloxy)-N,N-dimethylmethaniminium
hexachloroantimonate
SOMP 5-(succinimidyloxy)-3,4-dihydro-1-methyl-
2H-pyrrolium hexachloroantimonate
T3P 2-propanephosphonic acid anhydride
TATU O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetra-
methyluronium tetrafluoroborate
628 | Chem. Soc. Rev., 2009, 38, 606–631 This journal is �c The Royal Society of Chemistry 2009
TAPipU 1-(1-pyrrolidinyl-1H-1,2,3-triazolo[4,5-b]pyridin-
1-ylmethylene)pyrrolidinium tetrafluoroborate
N-oxide
TBFH N,N,N0,N0-tetramethylbromoformamidinium
hexafluorophosphate
TBTU O-benzotriazol-1-yl-1,1,3,3-tetramethyluronium
tetrafluoroborate
TCFH N,N,N0,N0-tetramethylchloroformamidinium
hexafluorophosphate
TCTU (2-(6-chloro-1-H-benzotriazol-1-yl)-1,1,3,3-
tetramethylaminium) tetrafluoroborate
TDBTU 2-(3,4-dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-
1,1,3,3-tetramethyluronium tetrafluoroborate
TEFFH tetraethylfluoroformamidinium
hexafluorophosphate
TFMS-DEP diethylphenyl(trifluoromethylsulfonyl)-
phosphoramidate
TFFH tetramethylfluoroformamidinium
hexafluorophosphate
TNTU 2-(5-norbornene-2,3-dicarboximido)-1,1,3,3-
tetramethyluronium tetrafluoroborate
TOTT S-(1-oxido-2-pyridinyl)-1,1,3,3-tetra-
methylthiouronium tetrafluoroborate
TODT S-(1-oxido-2-pyridinyl)-1,3-dimethyl-1,3-tri-
methylenethiouronium tetrafluoroborate
TOTU O-(cyano(ethoxycarbonyl)methylenamino)-
1,1,3,3-tetramethyluronium tetrafluoroborate
TPTU 1-((dimethylamino)(dimethyliminio)methoxy)-
2-hydroxypyridinium tetrafluoroborate
TSTU 2-succinimido-1,1,3,3-tetramethyluronium
tetrafluoroborate
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